The present invention relates to a semiconductor integrated circuit and, more particularly, to a semiconductor integrated circuit having a function which prevents malfunctioning of the circuit, arising from power source noises.
During the time a semiconductor integrated circuit is operating, a current flows through the power source lines of the circuit (i.e., a positive power source line and an earth line). When the current is large, the power source potential fluctuates. Such a fluctuation of the power source potential is called a power source noise since it causes a malfunction of the circuit.
In the case of a semiconductor integrated circuit, such as a semiconductor memory in which input/output data is constituted by a number of bits, the power source noise is generated when the "1" level is output from each output buffer, in order to read out the stored data, or when the "0" level is output from each of the output buffers. When the "1" level is output from each of the output buffers, this causes a delay in the timing at which the charges are supplied from the positive power source line to a load, with the result that the potential of the positive power source line quickly deteriorates. On the other hand, when the "0" level is output from each of the output buffers, the potential of the earth line increases rapidly. Since a capacitive component exists between the positive power source line and the earth line, when the potential of the positive power source line suddenly decreases due to an influence by the coupling, the potential of the earth line also decreases. Conversely, when there is a rapid increase in potential of the earth line, this brings about a corresponding increase in the potential of the positive power source line.
Since, in the case of a semiconductor integrated circuit, all the circuits (input circuit, internal circuit, output circuit) which are formed on a chip are connected to a common power source line, the power source noise which is generated through the driving of the output buffers change the level at which the input signal is detected in the input circuit. Thus, even if the input signal contains no noise, the input circuit operates as if the signal contains noise.
When power source noise, which will lower the potential of the positive power source line and that of the earth line, is generated while an input signal at the "0" level is being supplied to the input circuit, the potential of the input signal apparently rises, and the input circuit may detect this signal as one at the "1" level. Conversely, when such power source noise is generated while an input signal at the "1" level is being supplied to the input circuit, the potential of the input signal apparently falls, and the input circuit may detect this signal as one at the "0" level.
The input circuit is made of a Schmitt trigger circuit, in order not to make such an erroneous detection of the input signal. The schmitt trigger circuit has two threshold voltages Von and Voff different from each other. The first voltage, Von is used to detect that the input signal rises from the "0" level to the "1" level, and the second voltage, Voff, is used to detect that the input signal falls from the "1" level to the "0" level. As is well known, a Schmitt trigger circuit has a hysteresis input-output characteristic.
When the input circuit is a Schmitt trigger circuit, the following advantage can be obtained. That is, it does not malfunction when the power source noise is substantially smaller than its hysteresis voltage (=Von-Voff). The higher the hysteresis voltage, the greater noise the input circuit can withstand. If the hysteresis voltage of the input circuit exceeds a certain value, however, the operation margin to the input signal will decrease. Therefore, the hysteresis voltage cannot be above said value. It follows that the input circuit will malfunction when the power source noise is too large.